1. Field of the Invention
The present invention relates to an operational amplifier, and more particularly to an arrangement for canceling offset of an operational amplifier having a feedback loop for gain adjustment.
2. Description of the Background Art
In order to suppress the offset involved in an operational amplifier, the conventional practice has been directed to increasing the area of the gate electrode of transistors constituting the operational amplifier so as to render the effect on its offset of variation in manufacturing process smaller.
As a solution of canceling the offset of an operational amplifier without increasing the gate area of transistors constituting the operational amplifier, an offset canceller has been proposed in, for example, U.S. Pat. No. 6,448,836 to Kokubun et al. The offset canceller is designed to cancel the offset in an amplifier to develop an output signal corresponding to an input signal.
In more detail, when a voltage is applied to the input terminal of an operational amplifier, the switching elements are changed over to cause a voltage involving offset to be stored in the capacitive element. The switching elements are then changed over to cause feedback such that the gate voltages of the operational amplifier will be of the same value, based on the voltage stored in the capacitive element, thereby canceling the offset of the operational amplifier.
However, in an operational amplifier circuit having its feedback elements, such as capacitive or resistive elements, for setting the amplification factor of the operational amplifier provided outside of the operational amplifier, it is necessary to correct the amplification factor in order to overcome the offset ascribable to the capacitance and resistance values. With the conventional offset canceller, however, it is difficult to deal with such a situation with ease.
Additionally, with the conventional offset canceller, it is not possible to output a signal during the offset canceling operation. Stated differently, the canceling operation needs to be carried out only after setting the offset cancel mode, thus affecting the operation efficiency.